docs: def true_ces(subsystem, previous_state, next_state):
      """Set of all sets of elements that have true causes and true effects.
  
      .. note::
          Since the true |CauseEffectStructure| is always about the full system,
          the background conditions don't matter and the subsystem should be
          conditioned on the current state.
      """
      network = subsystem.network
      nodes = subsystem.node_indices
      state = subsystem.state
  
      _events = events(network, previous_state, state, next_state, nodes)
  
      if not _events:
          log.info("Finished calculating, no echo events.")
          return None
  
      result = tuple([event.actual_cause for event in _events] +
                     [event.actual_effect for event in _events])
      log.info("Finished calculating true events.")
      log.debug("RESULT: \n%s", result)
  
      return result